The Riddle of the Hollow Tree Enid Blyton
When Nick and Katie lose their parents in a car accident they are distraught to be living with their cruel Aunt Margaret and weak Uncle Charlie, but happy to still be together. But soon Aunt Margaret has had enough and, threatened with being separated in different foster homes, Nick and Katie hatch an ingenious plan (and prolonged adventure) with their friend Laura and her mischievous dog Russet.